The need for radio communications for mobile vehicles has greatly increased. In addition to passive radio receivers (e.g., AM/FM receiver or the like), two-way communications devices (e.g., CB, HAM, VHF, Cellular Telephone or the like) have been increasingly used in both land and marine applications. Satellite communications systems have been applied for both navigation (e.g., GPS or the like) and communications (e.g., Satellite telephony). Thus, there is an increasing demand for high performance antenna systems for mobile land and marine use.
Directional and omnidirectional antennas for use in satellite communications are known in the art. One example of such an omnidirectional antenna is disclosed, for example, in co-pending U.S. application Ser. No. 08/058,079 filed on May 10, 1993 entitled "MSAT MAST ANTENNA WITH REDUCED FREQUENCY SCANNING". Other types of directional antennas are also known in the art, including but not limited to parabolic dish antennas, capacitive array antennas, and other types of mechanically or electrically steered directional satellite antennas.
Such antennas may be used with a high degree of reliability for mobile land use for vehicles such as automobiles, trucks, or the like. In mobile land use applications, a relatively omnidirectional antenna may be used to compensate for vehicle movement. Alternately, vehicle movement may be compensated for by providing active or passive antenna positioning techniques.
An example of one such active antenna positioning device using a parabolic dish design is shown in Suzuki et al. U.S. Pat. No. 4,725,843, issued Feb. 16, 1988, shown in FIG. 3. FIG. 3 shows a vehicle 3 with parabolic dish antenna 1 and feed horn 2. As can be readily ascertained from FIG. 3, the relatively large dish antenna 1 precludes the use of any rooftop accessories (e.g., roof rack or the like) and presents quite a profile to the wind. In addition, such a design is somewhat aesthetically displeasing, thus precluding mass consumer acceptance. Mobile satellite communications systems have consumer applications and as such, a pleasing aesthetic design is a necessary criteria. The parabolic dish 1 of FIG. 3 includes a positioning mechanism to constantly reposition dish 1 as vehicle 3 travels. While such a positioning system may adequately compensate for the relatively slow direction changes experienced in an automobile, such a system may not be able to adequately compensate for the sudden and rapid movements of a marine vehicle, such as a small craft. Such a positioning system may also be relatively complex and fragile, and as such, constant repositioning may quickly reduce the service life of motors, gears, and the like.
For marine applications, the parabolic dish of Suzuki has particular disadvantages. As discussed above, the dish presents a large profile to the wind and thus would be susceptible to wind loading. For small craft use (e.g., boats under 50' in length) the additional wind loading due to the satellite dish may be entirely unacceptable. In addition, although such a dish may be applied to a large marine vessel (e.g., ocean liner, oil tanker, freighter, or the like) having a relatively stable or slowly oscillating motion characteristic, such an antenna could not readily be applied to small craft where rapid oscillations and changes in direction take place and may not suitably compensated for by active positioning techniques.
Various types of passive gimbaled mounts have been tried for marine applications. An example of such a gimbaled mount is shown, for example, in Akiyama published Japanese patent application 55-141804, published Nov. 6, 1980 and shown in FIG. 4. Akiyama uses a mount 9 having gimbaled supports 6 and 7 and a counterweight 4 to maintain position of an antenna 1 to compensate for the pitching and rolling of a ship. While such a design may be suitable for large ocean going vessels, the design presents a relatively large profile and thus increased wind loading. In addition, the pendulum design of Akiyama, while suitable for compensating for gentle pitching and rolling, may be unsuitable for the sudden and rapid oscillations which occur in small craft. In particular, such a pendulum, once placed in motion, will tend to oscillate due to its inherent inertia.
Other antenna mounting devices have been tried incorporating dampening features to dampen unwanted pendulum effects. One such device, Elston published UK patent application 2,127,622, published Apr. 11, 1984 is shown in FIG. 5. Elston shows an antenna mount having a bob weight 25 and a counterweight 29 containing a series of compartments filled with a fluid medium such as water or mercury. A sensor detects complex motions of the pendulum system and uses a motor to drive bob weight 25 up or down. In an alternative embodiment, a second pendulum 30 is provided to change the center of gravity of the device and compensate for the actions of bob weight 25. While such a system may be suitable for a large ocean going vessel, the size and complexity of such a device, as well as the cost, may make it unsuitable for small craft applications.
FIG. 6 shows a radar antenna mount disclosed in DeSatnick et al. U.S. Pat. No. 5,111,212, issued May 5, 1992. DeSatnick et al. provides an antenna mount for a sailboat which provides single axis rotation to compensate for heeling of the sailboat. DeSatnick et al. prevents unwanted pendular oscillations by providing a viscous damping fluid within the rotary mount. A series of vanes are provided in the dampening fluid such that, as the shaft rotates the viscus fluid passes between regions of the vanes and dampens the rotational movement. Although the device of DeSatnick et al. may be applied to small craft such as sailboats, the apparatus provides motion compensation in one direction only. For directional satellite antenna use, compensation must be provided for all axes. In addition, in powerboat applications, sudden and rapid changes in direction and wave induced oscillations may occur. Thus, the device of DeSatnick et al., which designed to compensate for the gentle heeling of a sailboat, may be inappropriate for satellite communications in power boat or other small craft applications.
In addition, the DeSatnick devices uses viscous fluid in much the same manner as a shock absorber, to dampen motion by metering the fluid between vanes through an orifice or port (See, e.g., Col. 3, lines 35-49). Such an arrangement serves not only to dampen oscillation, but also to dampen any rapid response to sudden shocks or accelerations. Thus, the apparatus may be unsuitable for small craft operation. In addition, the DeSatnick device requires a shaft opening with an appropriate seal in order to transfer the motion of the antenna to the vanes in the viscous fluid. In a harsh marine environment, such seals may be exposed to severe conditions, and thus presents an additional maintenance item and potential leakage condition.
Thus, it remains a requirement in the art to provide an antenna mount, particularly for small craft marine applications, which can compensate for sudden and rapid changes in vehicle direction as well as rapid oscillations while presenting a small profile for wind loading.